Disk drives are one type of peripheral storage devices commonly employed within digital computing systems. Such drives have the advantage of considerable storage capability and reasonably rapid random access to blocks of information stored on the data storage surface of a rotating disk within the drive.
While manufacturing processes and techniques for making magnetic data storage disks have improved dramatically, such processes still result in the production of magnetic data storage disks having a certain number of defects in the magnetic medium forming the data storage surface. The defects may be minute pinholes, or asperities caused by failures of the coating, plating, or sputtering processes used to apply the medium to the disk surface or by the embedding of foreign particles in the coating, etc. Whatever the cause or characteristic, a defect is a physical characteristic of a data storage disk which interferes with and even prevents the storage of useful information at the situs thereof.
While magnetic data storage disks usually are characterized by a multiplicity of concentric data tracks, it is known, particularly with optical disk data storage, to use a single spiral track. Whether a single spiral track, or a multiplicity of concentric tracks ar defined, subdivision of the track or tracks into predetermined segments of finite duration, called "sectors" is a general convention. Typically, a concentric data track of a fixed disk drive may include 17 data sectors, with each sector capable of containing a certain amount of user data, such as 512 bytes. Each sector also includes certain overhead information which is needed by the disk drive subsystem for proper operation, but which is unavailable to the user. This overhead typically includes a sector preamble containing information for flagging the beginning of the sector to the disk drive controller and for identifying the sector, typically by cylinder number, head (surface) number, and sector number. One typical arrangement of data within a sector is set forth in FIG. 20 of commonly owned U.S. Pat. No. 4,639,863, the disclosure of which is hereby incorporated by reference.
Immediately preceding the sector identification data is a unique signaling arrangement or pattern we refer to as an "address marker sequence". The address marker sequence typically includes a constant pattern multi-byte preamble, a sync byte, and an address mark byte. The controller constantly looks for this address marker sequence whenever data is being read back from the data surface. The controller requires this information to establish phase lock to the data rate and then to recover the data located in the sector, including the sector identification data.
Since the only way available to the controller to find a sector of interest is to read its address marker sequence, it is very important that no media defects be located within the nominal address marker sequence region and the immediately following sector identification field. Herein, this area of each sector is defined as the non-assignable region ("NAR"), and there is a NAR for each sector within each track on every data surface of the disk drive. This NAR includes the nominal address marker sequence region, and it is further increased by a margin which is related to spindle speed tolerance, index drift and analog media test equipment defect location accuracy. The required margin is a function of the worst case, and spindle speed tolerance typically varies from a small value (e.g. 5 bytes) to a large value (e.g. 150 bytes) as the distance of the sector location extends away from a fixed reference point, such as a spindle index clocking signal put out once each revolution of the disk. Thus, in this example the worst case is 150 bytes, and this value becomes the controlling margin.
So important has been the need not to encounter a defect within a NAR that the prior practice has been to map out an entire data track if it contains a single defect, irrespective of its location. Thus, in the instance of one media defect, elimination of a 17 sector data track containing the defect results in loss of 8704 bytes of data storage capability. More recently, the inclusion of a spare sector within each track has enabled reduction of the number of full tracks lost, but the swapping of a spare sector for a data sector has been possible only in those situations wherein a media defect did not fall into a NAR. If a single media defect lay in a NAR, the entire track was lost, since it would not be possible to read the sector identification data and know that this sector is to be skipped over, and the spare sector substituted in its place.
While data tracks having 17 sectors have become a de facto industry standard for small computing systems, such as the IBM Personal Computer.TM., for example, the data storage capability within a data track has increased, to the point that it is practical now to include as many as two full 17-sector logical tracks within a single physical track. This approach is taken in an increased data storage capacity disk file subsystem developed under the aegis of the assignee of the present invention and as set forth in U.S. patent application Ser. No. 07/052,709, filed on May 20, 1987, for "Modular Unitary Disk File Subsystem Having Increased Data Storage Capacity", the disclosure of which is hereby incorporated by reference.
Another prior art approach, which is described in U.S. Pat. No. 3,997,876, calls for the placement of a special gap in the data track directly over the defect in a manner which will then be transparent to the storage system. However, this approach required a considerable overhead of special electronics circuitry and further required data storage overhead within the data track in terms of a defect field in each sector which was read in order to locate the special gap thereby to ignore it. Also, the placement of a special gap over the defect raised the possibility of timing errors during data recovery operations with a conventional disk drive controller, since a suspension of data signals from the disk during the gap may cause the phase locked oscillator in the data separator circuit of the controller to lose synchronism.
A hitherto unsolved need has arisen for an improved method for shifting a predetermined data track soft sectored format by a predetermined amount in order move a media defect from a NAR into an area that may be mapped out of use.